1. Field of the Invention
This invention relates to a method for detecting and controlling the air-fuel ratio in an internal combustion engine, more particularly to a method for detecting the air-fuel ratio in a multiple cylinder internal combustion engine accurately and controlling to a target air-fuel ratio with good convergence.
2. Description of the Prior Art
It is a common practice to install a single air-fuel ratio sensor constituted as an oxygen concentration detector in the exhaust system of a multiple cylinder internal combustion engine and feedback the detected value for regulating the amount of fuel supplied to a target air-fuel ratio. A system of this type is taught by Japanese Patent Publication No. Sho 59(1984)-101562, for example.
In the system, in order to improve the detection accuracy, a time lag counted from a reference timing (a first cylinder's TDC position) and required for the exhaust gas flowing out of the individual cylinders to reach the air-fuel ratio sensor is predetermined in advance in response to the operating condition of the engine. And taking the predetermined time lag into consideration, the air-fuel ratio is detected for the individual cylinders and is feedback controlled to a target value. However, since the air-fuel ratio sensor constituted as an oxygen detector is arranged to detect the air-fuel ratio through a generated electromotive force caused by a chemical reaction which occurs when an element of the oxygen detector comes into contact with the exhaust gas, the sensor can not respond immediately and there is a delay in detecting the air-fuel ratio after the exhaust gas has reached the sensor. This means that, until the delay has been solved, the air-fuel ratio of the burnt mixture could not be detected precisely and hence the accurately and excellent convergence could not be expected in the air-fuel ratio feedback control.